Concrete is one of the most consumed substances with two billion tons in annual production, involving at least 5% annual anthropogenic global CO2 emission. As a building material, concrete offers high compressive strength at low cost; however, it suffers from low tensile and flexural strength. Thus, concrete generally requires bulky cross-sectional geometries, sometimes in combination with pre-stressed or post-tensioned reinforcing, to minimize potential cracking Even when such measures are used, the resulting product remains highly susceptible to compromised durability. Such durability issues have been magnified during the last decade through the aging infrastructure (nearing end of design life) of the United States. The recent catastrophic bridge collapses in Pennsylvania (partial) and Minnesota (complete) have refocused attention toward structural evaluation and rehabilitation. Over $1.5 trillion is estimated to be required to perform repair and restoration of concrete structures for achieving acceptable levels of safety and function. This large restoration effort warrants advanced materials to ensure longer lifespan for new construction as well as for repair of existing structures, thereby minimizing resurgent rehabilitation cost. Other applications for concrete also suffer from the inherent brittleness of the material and manifest themselves in high rates of rejection of the finished product. Such applications include, but are not limited to, cellular concrete, autoclaved aerated concrete, and many others.
Existing methods for structural restoration include physical repair by external plate attachment and chemical attachment through patching. Patching, one of the popular techniques for localized damage repair, uses cement mortar, enhanced with both inorganic (e.g., fumed silica and magnesium phosphate) and polymeric admixtures. Pre-made polymer fiber reinforcements are also used for conditioning of damaged concrete surfaces and components. However, most research has concentrated on improving cement mortar patch properties to provide improved durability of the patches. In situ polymerization during mortar hardening is known to have remarkable chemical advantages, particularly through the polymer-cement bonding via the active ions in cement. The polymer choice in the cement matrix is thus driven by this mutual compatibility between the polymers and the cement matrix.
Relatively better tensile properties of the polymer fibers (compared to that of cement) and the functional groups along the polymer backbone, for effective binding with cement cations (e.g., Ca2+ and Al3+) encouraged the use of polymer as additives in structural repair. The selection of polymer is dominated by the chemical compatibility and cement workability. Esters, vinyl-alcohols, acetates, styrenes, epoxy, and synthetic rubber are the most commonly used polymers due to their inherent chemical compatibility properties. The other advantage of these polymer admixtures is low water permeability which increases corrosion and freeze-thaw resistance of concrete structures. Moreover, relatively faster curing time for polymer enhanced concrete has allowed for their widespread marketization and usage in floorings and pavements, waterproofing, adhesion, decorative coatings, bridge deck coverings, anticorrosive coatings, etc. However, most polymer enhanced cementitious applications require high polymer:cement ratios and thus leave the challenge of inflated cost.
Though the polymeric admixtures allow for better integration of the patchwork with the existing damaged structure, durable and effective patching requires high amounts (10-20 wt % of cement) of polymer addition. Such high amounts of polymer necessitate addition of other admixtures to maintain proper workability and to minimize air-entrainment. Such chemical additions result in exorbitant cost and this is noted as the primary challenge in making the technology more effective. The cost of repair limits its applicability in large scale structures, such as, pavements, foundations, and substructure and superstructure components for bridges. Moreover, most polymers used in polymer-enhanced mortar are known to elicit toxic response to aquatic and biologic environments and are also resistant to biodegradation. High amounts of polymer in repair mortar pose significant environmental challenges.
The emergence of polymers from biological origin with higher stiffness and tensile properties presents a novel area for research on polymer-enhanced concrete and mortar.